The chemical curing of epoxidized diene polymers is of interest for sealant, coatings, and adhesives applications. U.S. Patent 5,229,464 describes low molecular weight epoxidized diene block copolymers and states that they may be crosslinked by the addition of muftifunctional carboxylic acids and acid anhydrides. U.S. Pat. Nos. 5,478,885 and 5,461,112 describe similar polymers which are used as toughening modifiers for epoxy resins. These epoxidized diene polymers are shown to be curable with carboxylic acid or anhydride curing agents. In this latter application, partially and fully saturated aliphatic carboxylic acids or anhydrides have been found to be very useful and to allow the production of very good products. However, a number of problems have been encountered when the same partially or fully saturated aliphatic carboxylic acids or anhydrides have been used as curing agents to cure the epoxidized diene polymers when epoxy resins are not present in the compositions to be cured. Such is the case when these polymers are used in sealant, coatings, and adhesive applications.
I have discovered that when such carboxylic acid anhydrides are used to cure these epoxidized diene polymers alone, bubbles form in the crosslinked product. Relatively low conversion levels are achieved with these curing agents and the products exhibit low strength because of both a low level of crosslinking conversion of the epoxy and defects introduced into the product as bubbles.
It has been found that the bubbles are produced as a result of the evolution of carbon dioxide during the decarboxylation of the anhydride. This side reaction competes with the desired crosslinking reaction and the kinetics are such that it takes up enough of the epoxy groups to significantly lessen the total curing while producing CO.sub.2 bubbles in the product. This problem does not occur to any noticeable extent in the epoxy resin impact modification as described in U.S. Pat. Nos. 5,478,885 and 5,461,112 above because the reaction of the anhydride with the epoxy resin is much faster than the reaction between the epoxidized diene polymer and the anhydride and also because in that patent, the majority of the composition is the epoxy resin.
It is dear from the above that when there is a desire for a high strength, defect free sealant, coating, or adhesive utilizing an epoxidized diene polymer, a curing agent different from the partially and fully saturated aliphatic carboxylic acid anhydrides is needed. There are other types of curing agents which can be used, but acid anhydride curing agents are preferable in the curing of epoxidized diene polymers because amines will not rapidly cure such polymers without an epoxy resin present. The Applicant here has found that certain aromatic acid anhydride curing agents are capable of curing epoxidized diene polymers to a high degree of cure and high strength without the formation of bubbles.